In general, an ink for a ballpoint pen includes a water based ink for a ballpoint pen comprising water or water-soluble solvent and having a viscosity of as low as 10 mPa.multidot.s or less, and an oil based ink for a ballpoint pen comprising an oil based solvent such as a mineral oil, polyhydric alcohol, fatty acid and Cellosolve and having a viscosity of from 1000 to 20000 mPa.multidot.s.
A ballpoint pen using an oil based ink for a ballpoint pen has such structure that the ink adhered on a ball at the pen point through an ink reservoir having a small diameter is transferred onto a paper surface by the rotation of the ball, and the transferred amount of the ink is fed to the pen point through the reservoir.
A ballpoint pen using a water based ink for a ballpoint pen has such structure that an ink is fed to a ball surface and a paper surface by means of capillary action of a feed prepared by binding fine fibers.
The above described water based ink and oil based ink for a ballpoint pen each have excellent advantages, but also have various problems, respectively.
For example, since the water based ink for a ballpoint pen has low viscosity, the ink is fed by means of utilizing a principle of capillary action. That is to say, if the tip portion of the ballpoint pen and paper are simply brought into contact, the capillary action works at the contact point to feed the ink to the pen point. Therefore, favorable writing is made on the paper without applying so higher writing pressure, so that splitting, starving and blobbing are hardly caused. On the contrary, if the ink is directly filled in the ink reservoir, the ink tends to seep due to vibrations, impacts or a temperature rise of the open air, and the ink volume to be fed to the ball at the pen point becomes unstable. Therefore, the ballpoint pen requires complicated structure having a feed prepared by binding fine fibers. Moreover, there is another problem in that it is difficult to check the amount of the ink reserved in the ink reservoir.
On the other hand, since the oil based ink for a ballpoint pen has a high viscosity, it has such characteristics that blobbing of the ink from the pen point can be prevented, that the ink can be directly filled in the ink reservoir having a small diameter to simplify the structure of the ballpoint pen and that the remaining amount of the ink can be checked by using a transparent material for the ink reservoir. On the contrary, it has such disadvantages that since the ink is transferred only on the paper surface contacted with the rotating ball, if the rotation of the ball exhibits an unstable behavior, splitting and starving are easily caused and that since the ink hardly penetrates to the paper surface, blobbing which causes stain with the untransferred ink is easily caused.
To solve the above problems, recently, a water based ink for a ballpoint pen in which a gelling agent or a water-soluble paste is added to the water based ink to impart a special viscosity property (hereinafter referred to as a "pseudoplastic water based ink") has been widely known.
With regard to the ballpoint pen using this pseudoplastic water based ink, shear force is added to the ink due to the rotation of the ball at the point of the tip at the time of writing, so that the ink viscosity decreases so as to enable smooth writing as is the case with a ballpoint pen using water based ink and a favorable writing can be made on the paper surface. Moreover, when the pen is not used for writing, since the viscosity of the ink is high, blobbing of the ink from the pen point can be prevented, and the ink can be directly stored in the ink reservoir to simplify the structure of the ballpoint pen, and the remaining amount of the ink can be easily checked by using a transparent material for the ink reservoir. As described above, when a pseudoplastic water based ink for a ballpoint pen is used, a writing instrument which has both of characteristics of the ballpoint pen using a water based ink or the ballpoint pen using an oil based ink can be provided. When a pseudoplastic water based ink for a ballpoint pen is actually used, however, it is very difficult to maintain the intended quality of the pen, unless the viscosity of the ink is adjusted depending upon the used tip. For example, when an ink having a quality suitable for a tip for a small-diameter ball is used with a tip for a large-diameter ball, since the clearance between the ball and the holder varies and the shear speed imparted to the ink at the time of writing becomes small, blobbing and splitting may be caused, and there is another problem in that the dryness of lines is decreased due to the excessive flow of the ink.
It is an object of the present invention to solve the above mentioned problems and to provide a pseudoplastic water based ink for a ballpoint pen which can be adaptable to a tip in any condition of the ball diameter, materials, dimensions and the like, and which does not cause phenomena such as blobbing and splitting and can be fed in a stable amount for smooth writing at any time, and which has high stability with the lapse of time and high dryness of lines.